Tissue abnormalities involving the skin usually are detected and assessed by a combination of visual inspection and palpation. In certain clinical situations the sensitivity of the visual inspection can be enhanced by the use of non-white light (either ultraviolet or a narrow band in the visible), or by the prior application of a contrast-enhancing agent such as dilute acetic acid or certain stains. Tissue abnormalities that involve surfaces that cannot be palpated (such as the bronchi or the urinary bladder) may be visualized via an appropriate scope. Some specialized scopes can detect induced fluorescence. If the abnormality in question is associated with a difference in either the extend or the pattern of tissue vascularization, such a scope may be used to determine the limits of the area involved by the abnormality, by visualizing an injected bolus of fluorescein as it passes through the vasculature of both the lesion and the adjacent normal tissue.
In addition, fluorescence-detecting scopes are being used experimentally to identify areas of tissue that show strong porphyrin fluorescence following the intravenous injection of exogenous porphyrins such as hematoporphyrin IX (HpIX), hematoporphyrin derivative (HpD), or "dihematoporphyrin ether" (Photofrin II). Such porphyrins tend to accumulate semi-perferentially in malignant tissues, but they also accumulate in tissues that are regenerating following an injury or in the rapidly growing tissues of an embryo or fetus. Normal liver, spleen, and kidney also tend to accumulate these porphyrins. Using such compounds and fluorescence-detecting scopes, areas of malignant tissue too small to be identified by standard forms of visual inspection have been identified in the bronchi and in the urinary bladder.
Unfortunately, a clinically significant (photosensitizing) amount of porphyrin persists in the skin for at least two weeks, (occasionally for more than two months) following the intravenous injection of HpIX, HpD, or Photofrin II. This means that patients must avoid exposure to sunlight (either direct, or through window glass) for an annoyingly long period of time post-injection. Understandably, patient compliance often is poor, and accidental phototoxic "sunburn" is a common occurrence in the weeks following a diagnostic or therapeutic injection of porphyrin. Persistent photosensitivity is the major hazard associated with this technique, and is the main reason why it is not used more widely.
The standard or conventional forms of treatment for cancer are surgery, radiotherapy and chemotherapy. However, other forms of treatment are also known, including photochemotherapy or photodynamic therapy (PDT). PDT is currently being used, on an experimental basis, to treat several different types of cancer as well as certain nonmalignant lesions such as psoriasis. The patient is given a photo-activatable drug that has some degree of specificity for the tissue being treated. A tissue volume that includes the target tissue is then exposed to photoactivating light so as to destroy the target tissue while causing only mild and reversible damage to the other tissues in the same treatment volume.
There are two main types of photochemotherapeutic agents in clinical use at present. The first type, methoxypsoralens, are given systemically. Ultraviolet light is essential to activate them. Localized exposure of psoralen-containing tissues to ultraviolet light induces a localized photochemical reaction that causes the drug to bind covalently to the DNA of living cells, thus destroying their proliferative potential. The second type, porphyrins, are also given systemically (by intravenous injection), although occasionally they are given either topically or by intralesional injection. They can be activated by visible (red) light. The localized exposure of porphyrin-containing tissues to such light does not induce a chemical reaction between cell components and the porphyrin molecules. Instead, the porphyrins act as catalysts by trapping the energy of the photoactivating light and then passing it on to molecules of oxygen, which in turn are raised to an excited state that is capable of oxidizing adjacent molecules or structures. Cell death is not caused primarily by damage to the DNA, but by damage to essential membrane structures.
Photochemotherapy is used at present for the treatment of certain types of cancer and non-malignant lesions, including psoriasis. The goal of such treatment is sometimes cure (mainly for basal cell carcinomas), but usually the goal is palliation through local control when none of the standard forms of therapy are considered likely to offer a significant degree of benefit to the patient.
Methoxypsoralen (PUVA) therapy is used mainly for the treatment of psoriasis, but sometimes it is also used to treat very superficial cancers that involve the skin (mainly mycosis fungoides). However, there are two serious problems with such treatments. First, the procedure has been demonstrated in humans to be carcinogenic. Second, the photoactivating ultraviolet light is absorbed so strongly by most tissues that the depth at which malignant tissue can be killed is limited to a few millimeters below the illuminated surface. These problems severely limit the usefulness of the methoxypsoralens for photochemotherapy.
At present, the porphyrins most commonly used for photochemotherapy are Hematoporphyrin IX (HpIX), Hematoporphyrin derivative (HpD), and Photofrin II, a semipurified form of HpD. When porphyrins are used as photosensitizers, cell death results from damage to essential membrane structures rather than from damage to DNA. Consequently, malignant transformation is not a serious problem. Moreover, since the visible (red) light that is used to photoactivate porphyrins penetrates tissue much more deeply than does the ultraviolet light that must be used to photoactivate methoxypsoralens, the depth at which porphyrin-treated tissue can be killed is substantially greater. Also, since certain types of porphyrins show a significant tendency to accumulate preferentially in malignant tissues, it is sometimes possible to destroy malignant tissue without causing clinically significant damage to adjacent normal tissues.
The main problem with the systemic use of HpIX, HpD and Photofrin II is that photosensitizing concentrations persist in the skin for several weeks to several months following their administration. Consequently, severe accidental phototoxic skin reactions may occur unless the patient avoids exposure to sunlight (either direct, or filtered through window glass) until the concentration of the photo-sensitizer in the skin has been reduced to a harmless level. At present, the problem of photosensitivity following the administration of porphyrins is handled by advising the patient to avoid any form of exposure to sunlight (or to very bright artificial lights) for a period of at least two weeks post-injection, and to initiate subsequent exposure to sunlight very cautiously. Not all patients comply with these instructions, since it often is quite inconvenient to do so. In addition, the use of a sunscreen with a high blocking factor is recommended with a warning that this will only reduce the hazard somewhat, not eliminate it completely. In a few cases, patients whose photosensitization persisted for more than a month post-treatment have been given large daily doses of beta-carotene over a period of several months in an attempt to prevent accidental photo-toxic damage. Finally, attempts have been made to reduce phototoxicity by applying the photosensitizer topically to a limited area.
However, another type of problem is encountered if HpIX or HpD is applied topically in DMSO (Dimethylsulfoxide) Azone, or some other vehicle intended to enhance their diffusion through tissue. The porphyrins tend to become immobilized wherever they happen to be when the DMSO Azone becomes diluted by normal tissue fluids to such an extent that the porphyrins can no longer diffuse through the tissue (or even remain in solution). Consequently, the topical application of porphyrins often is associated with a loss of specificity for malignant tissues, and normal tissues near the site of application may develop persistent photosensitization from the localized concentration of porphyrin.